1. Field of the Invention
The present invention relates to a hydraulic shock absorber.
2. Description of the Related Art
In a hydraulic shock absorber in which a damper cylinder is not provided in inner portions of an outer tube and an inner tube, there is a structure in which the inner tube is slidably inserted into the outer tube. A partition wall member is provided in an inner periphery of the inner tube. An oil chamber is defined at a lower side of the partition wall member. An oil reservoir chamber is defined in an upper portion of the partition wall member. A piston rod attached to the outer tube is slidably inserted into the inner tube through the partition wall member, and a piston coming into slidable contact with an inner periphery of the inner tube is fixed to a leading end portion of the piston rod inserted into the inner tube.
In the conventional hydraulic shock absorber, in Japanese Patent Application Laid-Open (JP-A) No. 2003-269515, as shown in FIG. 15, there is disclosed a structure in which an inner tube 2 is slidably inserted into an outer tube 1 via seal means 1A and 2A which are respectively fixed to an opening portion in an inner periphery of the outer tube 1 and a leading end portion of an outer periphery of the inner tube 2. An annular oil chamber 3 is surrounded by the inner periphery of the outer tube 1, the outer periphery of the inner tube 2 and these two seal means 1A and 2A is defined. A partition wall member 4 is provided in an inner periphery of the inner tube 2. An oil chamber 5 is defined at a lower side, and an oil reservoir chamber 6 is defined at an upper side. Further, in this hydraulic shock absorber, a piston 7A coming into slidable contact with an inner periphery of the inner tube 2 is fixed to a leading end portion of the piston rod 7 which is inserted into the inner tube 2. The oil chamber 5 is divided into a piston rod side oil chamber 5A in which the piston rod 7 is accommodated and a piston side oil chamber 5B in which the piston rod 7 is not accommodated. The annular oil chamber 3 is communicated with the piston rod side oil chamber 5A or the piston side oil chamber 5B via an oil hole 3A which is provided in the inner tube 2. A compression side damping force generating valve 8A and an extension side damping force generating valve 8B are provided respectively in a compression side flow path and an extension side flow path which are provided in the piston 7A and communicates the piston rod side oil chamber 5A with the piston side oil chamber 5B. Further, in this hydraulic shock absorber, a cross sectional area S1 of the annular oil chamber 3 is formed larger than a cross sectional area S2 of the piston rod 7. The partition wall member 4 is provided with a check valve 9A which blocks a flow from the oil chamber 5 to the oil reservoir chamber 6 at a time of an extension side stroke, and the partition wall member 4 is provided with a micro flow path 9B which passes through the oil chamber 5 and the oil reservoir chamber 6.
In the hydraulic shock absorber described in JP-A No. 2003-269515, a working fluid at an amount of an inserting volumetric capacity of the piston rod 7 going into the inner tube 2 in a compression side stroke is transferred to the annular oil chamber 3 from the oil chamber 5A in an inner periphery of the inner tube 2 via the oil hole 3A of the inner tube 2. At this time, since a volume increasing amount ΔS1 (a replenishing amount) of the annular oil chamber 3 is larger than a volume increasing amount ΔS2 of the piston rod 7, a shortfall of (ΔS1−ΔS2) a necessary replenishing amount of the oil to the annular oil chamber 3 is replenished from the oil reservoir chamber 6 via a check valve 9A. In this compression side stroke, a compression side damping force is generated on the basis of a deflecting deformation of the compression side damping force generating valve 8A.
Further, the working fluid in an amount of a going-out volumetric capacity of the piston rod 7 going out of the inner tube 2 in an extension side stroke is transferred to the oil chamber 5A in the inner periphery of the inner tube 2 from the annular oil chamber 3 via the oil hole 3A of the inner tube 2. At this time, since the volume reducing amount ΔS1 (a discharge amount) of the annular oil chamber 3 is larger than the volume reducing amount ΔS2 of the piston rod 7, a surplus amount of (ΔS1−ΔS2) in the discharge amount of the oil from the annular oil chamber 3 is discharged to the oil reservoir chamber 6 via the micro flow path 9B. In this extension side stroke, an extension side damping force is generated on the basis of a deflecting deformation of the extension side damping force generating valve 8B. Further, an extension side damping force is generated by a passage resistance of the micro flow path 9B.
In the hydraulic shock absorber described in JP-A No. 2003-269515, the working fluid in the piston side oil chamber 5B which is compressed by the piston 7A in the compression side stroke deflects and deforms the compression side damping force generating valve 8A, thereby generating a stable compression side damping force.
However, in the extension side stroke, the extension side damping force generated in the micro flow path 9B of the partition wall member 4 is caused by the setting of the passage resistance of the micro flow path 9B and a small amount of surplus oil amount of (ΔS1−ΔS2) mentioned above and passing through the micro flow path 9B, and is unstable. Therefore, in the hydraulic shock absorber described in JP-A No. 2003-269515, there is a problem in generation of a stable extension side damping force.
Further, in the hydraulic shock absorber described in JP-A No. 2003-269515, the pressure of the working fluid in the piston rod side oil chamber 5A which is compressed by the piston 7A in the extension side stroke is directly applied to the seal means 1A and 2A of the outer tube 1 and the inner tube 2. Accordingly, a maximum value of the extension side damping force is restricted on the basis of a relationship of tendency that friction is deteriorated by an application of a great tension force due to a self-seal effect of the seal means 1A to the inner tube 2, and a pressure application to the seal means 1A.
Further, in the hydraulic shock absorber described in JP-A No. 2003-269515, in the compression side stroke, an air chamber in the oil reservoir chamber 6 in the upper portions of the outer tube 1 and the inner tube 2 is compressed so as to serve as an air spring, however, since the pressure is applied as it is to the compression side damping force generating valve 8A, a hardness feeling is generated at the far side of the compression side stroke.